Spider bites

Overview

Spider bites are usually harmless. Many bites attributed to spiders turn out to have been inflicted by other bugs. Skin infections also have been mistaken for spider bites.

Only a few types of spiders have fangs long enough to penetrate human skin and venom strong enough to severely affect a human being. In the United States, these include the black widow spider and the brown recluse spider.

Black widow spider bites can cause severe abdominal pain or cramping. Brown recluse spider bites can cause a sting or sharp pain, like a bee sting. With severe bites, surrounding skin may die within a few hours. Both types of spiders generally live in undisturbed areas, such as attics or sheds. And they don't bite unless threatened.

Black widow spider

The black widow spider is known for the red hourglass marking on its belly.

Brown recluse spider

The brown recluse spider is known for the violin-shaped marking on its top.

Symptoms

Typically, a spider bite looks like any other bug bite — a red, inflamed, sometimes itchy or painful bump on your skin — and may even go unnoticed. Harmless spider bites usually don't produce any other symptoms.

Black widow spider bites

Signs and symptoms of a black widow spider bite may include:

Pain. Typically beginning within an hour of being bitten, pain generally occurs around the bite mark, but it can spread from the bite site into your abdomen, back or chest.

Cramping. Abdominal cramping or rigidity can be so severe that it's sometimes mistaken for appendicitis or a ruptured appendix.

Sweating. Excessive sweating can occur.

Brown recluse spider bite

The pain associated with a brown recluse spider bite typically increases during the first eight hours after the bite. You may also have fever, chills and body aches. The bite usually heals on its own in about a week. In a minority of cases, the skin at the center of the bite can become dark blue or purple and then evolve into a deep open sore (ulcer) that enlarges as the surrounding skin dies. The ulcer usually stops growing within 10 days after the bite, but full healing can take months.

When to see a doctor

Seek prompt medical care in the following situations:

You are unsure whether the bite was from a poisonous spider.

The person who was bitten experiences severe pain, abdominal cramping or a growing ulcer at the bite site.

The person who was bitten is having problems breathing.

Your doctor may recommend a tetanus booster shot if you haven't had one in the last five years.

Causes

Severe spider bite symptoms occur as a result of injected spider venom. The severity of symptoms depends on the type of spider, the amount of venom injected and how sensitive your body is to the venom.

Risk factors

Although dangerous spider bites are rare, your risk of being bitten increases if you live in the same areas that the spiders do and you happen to disturb their habitat. Both black widow and brown recluse spiders prefer warm climates and dark, dry places.

Black widow habitat

Black widow spiders can be found throughout the U.S. but more so in the southwestern states. They prefer to live in:

Sheds

Garages

Unused pots and gardening equipment

Woodpiles

Brown recluse habitat

Brown recluse spiders are found most commonly in the southern Midwest and in limited areas of the South. Recluses are so named because they like to hide away in undisturbed areas. They mostly prefer to live indoors, in places such as:

The clutter of basements or attics

Behind bookshelves and dressers

In rarely used cupboards

Outside, they seek out dark, quiet spots, such as under rocks or in tree stumps.

Complications

Very rarely, a bite from a black widow or brown recluse spider may be deadly, particularly in children.

Prevention

Spiders in general, including the black widow and brown recluse, bite only in defense, when being trapped between your skin and another object.

To prevent spider bites:

Learn what dangerous spiders look like and their preferred habitat.

Wear a long-sleeve shirt, hat, long pants tucked into socks, gloves and boots when handling stored boxes or firewood, and when cleaning out sheds, garages, basements, attics and crawl spaces.

Inspect and shake out gardening gloves, boots and clothing that have been unused for a while.

Use insect repellents, such as DEET or Picaridin, on clothing and footwear.

Keep insects and spiders out of the house by installing tightfitting screens on windows and doors, sealing cracks where spiders can come in and using safe indoor insecticides.

Remove piles of rocks or lumber from the area around your house and avoid storing firewood against the house.

Make sure beds aren't pushed against the wall and that only the legs of the bed touch the floor. Don't store items under the bed and don't let bedding drag on the floor.

Vacuum spiders and spiderwebs and dispose of them in a sealed bag outside to prevent re-entry into the house.

Diagnosis

Your doctor may suspect a spider bite based on your history and your signs and symptoms, but a specific diagnosis is difficult to confirm. Confirmation requires:

An eyewitness to the bite

Identification of the spider by an expert

Exclusion of other possible causes

Black widow identification

Some clues for identifying black widow spiders include:

Shiny black body

Large round abdomen

Red hourglass on underside of abdominal area

Central body about a half-inch (12 to 13 millimeters) long

Entire body, including legs, can be more than an inch (2.5 centimeters) across

Brown recluse identification

Some clues for identifying brown recluse spiders include:

Color ranges from yellowish tan to dark brown

Dark violin shape on top of the leg attachment segment

Six eyes — a pair in front and a pair on both sides — rather than the usual spider pattern of eight eyes in two rows of four

Central body is a quarter-inch to three-quarters of an inch (6 to 19 mm) long

Treatment

For most people with spider bites, including black widow and brown recluse spider bites, the following treatment measures are all that's required:

Clean the bite with mild soap and water. Apply an antibiotic ointment if you think the bite was caused by a brown recluse spider.

Apply a cool compress to the bite. This helps reduce pain and swelling.

If the bite is on an arm or leg, elevate it.

Take over-the-counter medications as needed. You might try a pain reliever, such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others), or an antihistamine (Benadryl, Chlor-Trimeton, others).

Observe the bite for signs of infection.

Your doctor may also recommend a tetanus booster shot if you haven't had one in the last five years. You may need antibiotics if the bite becomes infected.

Black widow antivenom

If a black widow bite is causing severe pain or life-threatening symptoms, your doctor may recommend an antivenom, which may be injected into a thigh muscle or given through a vein (intravenously). Antivenom can cause serious allergic reactions, so it must be used with caution.

Preparing for an appointment

If you've been bitten by a spider that you suspect is a black widow or brown recluse, call your primary care doctor or go to an urgent care center. If your doctor has online services, an option may be to email a photo of the spider to your doctor.

What you can do

To help your doctor understand your symptoms and how they might relate to a spider bite, you can:

Bring the spider or a photo of the spider with you

Write down any symptoms you're experiencing

Write down questions to ask your doctor

Some basic questions you might want to ask include:

If this is a dangerous spider bite, what's the next best step?

If this isn't a spider bite, what are possible causes for my symptoms or condition?

Do I need any tests?

How long will my symptoms last?

What is the best course of action?

What to expect from your doctor

Your doctor is likely to ask you a number of questions. Being ready to answer them may reserve time to go over any points you want to spend more time on. Your doctor may ask: